This invention relates to water-free separation membranes for separating aliphatically-unsaturated hydrocarbons from saturated hydrocarbons. In another aspect, the invention relates to a process for utilizing water-free membranes comprised of preformed membranes of polymeric material capable of chemically bonding positive metal ions wherein the membranes have been plasticized resulting from contacting of the preformed membranes with polyhydric alcohols. The invention is especially useful for separating ethylene from gaseous mixtures containing ethylene and other hydrocarbons, for example, ethane, methane and hydrogen in the absence of water.
Considerable commercial interest exists for separating various aliphatically-unsaturated hydrocarbons from mixtures of hydrocarbons. Aliphatically-unsaturated hydrocarbons are reactive materials that serve various roles in the chemical and hydrocarbon industry, generally as intermediates in chemical synthesis. A number of unsaturated hydrocarbons are employed as monomers in the formation of polymers and in this regard, olefins such as ethylene, propylene, butadiene and isoprene are well known. Aliphatic unsaturated hydrocarbons are most frequently available on a commercial basis in admixtures with other chemical compunds. These unsaturated hydrocarbon-containing streams are usually by-products of chemical synthesis or separation processes. When the hydrocarbon streams are liquid under normal conditions or can readily be liquified, ordinary distillation techniques are used to separate the hydrocarbon components provided they have sufficiently different boiling points for the process to be economically feasible. However, distillation may not be an attractive separation procedure when the hydrocarbon mixtures contain materials having close boiling points which is often the case with hydrocarbons of the same number of carbon atoms or having a difference of only one carbon atom. In these cases, different separation processes must be used which are frequently costly and involve operations such as solvent extraction, extraction distillation, cryogenics and the like.
When the aliphatically-unsaturated hydrocarbons and mixtures thereof with other similar boiling point hydrocarbons are in essentially the gaseous state at ambient temperatures and pressures, separation of the desired component from the mixture may be even more troublesome. In these situations, cryogenic processes may be used, however, frequently expense is prohibitive. These difficulties have created the need for use of semipermeable membranes for separating unsaturated hydrocarbons from saturated hydrocarbon mixtures.
Facilitated transport is a membrane process in which one species of a feed mixture is transported across a membrane preferentially by virtue of a component of the membrane interacting specifically and reversibly with that species. The facilitated aliphatically-unsaturated hydrocarbon transport process most thoroughly studied has been the separation of ethylene from its mixtures with saturated hydrocarbons. Unsaturated hydrocarbons have been known to interact with silver ions. The kinetics and equilibria of this reaction are such that it can be utilized to facilitate membrane transport. Use of metal ions, for example, silver ions has been taught for facilitated transport of unsaturated hydrocarbons; however, in all these teachings, the systems require water to both impregnate the membrane and to saturate the feed gas in order to prevent drying of the membrane. It is thought that the use of aqueous saturation is an absolute requirement for the success of the facilitated transport prooess, for example, the separation of ethylene from ethane. It is assumed that the reason for this need for the presence of water is that water acts as a solvent for the silver salts and/or as a plasticizer for the polymer matrix and/or provides the necessary electronic environment for the silver ion. The high volatility of water makes this requirement a major limitation to the process. Accidental drying of the membrane innerly irreversible destroys its separation capacity.
Combinations of liquid barrier permeation involving metal complexing techniques have been utilized in separating aliphatically-unsaturated hydrocarbons from mixtures of other hydrocarbon materials. These systems provide for a liquid barrier as a continuous, distinct or separate liquid phase adjacent to and in contact with a semipermeable film membrane which is relatively non-selective with respect to the passage of the components of the hydrocarbon feed mixture. In addition, systems are known which are directed to methods for separating various materials from mixtures of aliphatically-unsaturated hydrocarbons and other hydrocarbons involving the combined use of liquid barrier permeation and metal complexing techniques which exhibit selectivity for the unsaturated hydrocarbons. In these processes, the liquid barrier is an aqueous solution having dissolved therein metal ions which will complex with the component to be separated. The liquid barriers are employed in contact with semipermeable membranes which are essentially impermeable to the passage of the separation barrier. The selectivity and separation ability of these aqueous barrier, metal ion containing systems can be rendered ineffective over prolonged use due to the loss of water from the barrier membrane system. The selectivity and separation ability of these aqueous, metal ion membranes decreases upon drying, thus the need for addition of water, generally through mixture with the feedstream.